1. Field of the Invention
The present invention relates to a current-voltage non-linear resistor and method of manufacture thereof, whose chief constituent is zinc oxide (ZnO), that may be employed in an over-voltage protection apparatus such as a surge arrester (or arrestor) or surge absorber.
2. Description of the Related Art
Over-voltage protection apparatus such as surge arresters or surge absorbers are commonly employed in order to protect power systems or the circuitry of electronic equipment from abnormal voltages. Such surge arresters or surge absorbers possess a non-linear resistance body, showing insulating properties under normal voltage but showing low resistance when subjected to abnormal voltage: they are thus effective for suppression of over-voltage. Such current-voltage non-linear resistors are ceramic bodies whose main constituent is zinc oxide (ZnO); at least one or more metal oxides are added thereto as additives in order to obtain a voltage non-linear resistance characteristic, and the material is then subjected to mixing, pelletizing, molding and sintering. An insulating layer of electrical insulating material is formed on the side faces of this sintered body in order to prevent flashover from the side faces during surge absorption.
Also, with the economic recession in recent years, miniaturization and improved performance of the equipment constituting power transmission installations is being sought, in order to reduce transmission costs in power systems. Current-voltage non-linear resistors whose chief constituent is zinc oxide are employed in surge arresters on account of their excellent non-linear resistance characteristic. If the current-voltage non-linear resistors that are employed in such surge arresters can be made of high resistance type, the number of current-voltage non-linear resistors that are provided in a laminated fashion in the surge arrester can be reduced, making it possible to miniaturize the transmission equipment.
Current-voltage non-linear resistors have been disclosed wherein, in order to make the current-voltage non-linear resistor of high resistance type, for example the content of auxiliary constituents such as Bi2O3, CO2O3, MnO, Sb2O3 and NiO is restricted, and wherein, furthermore, the crystalline phase of the Bi2O3 contained in the sintered body whose chief constituent is ZnO is restricted. An example is Laid-open Japanese Application No. Tokkai 2001-307909 (hereinbelow referred to as Patent Reference 1). With such current-voltage non-linear resistors, high resistance and an excellent non-linear resistance characteristic can be obtained.
Also, current-voltage non-linear resistors have been disclosed that are of high resistance and in which an excellent non-linear resistance characteristic is obtained by addition of rare earth oxides in a current-voltage non-linear resistor whose chief constituent is zinc oxide and to which Bi2O3, CO2O3, MnO, Sb2O3 or the like have been added. Examples are Published Japanese Patent Nos. 2904178, 2933881, 2940486, and 3165410 (hereinbelow referred to as Patent References 2 to 5).
The technique has also been disclosed of obtaining current-voltage nonlinear resistors having a high resistance and excellent temporary overvoltage energy absorption capability, by employing solely rare earth raw materials having a grain growth suppressing effect to prepare very fine-grained powder beforehand. An example is to be found in Laid-open Japanese Patent Application No. Tokkai 2001-233668 (hereinbelow referred to as Patent Reference 6).
Also, in recent years, in order to achieve sufficient miniaturization of surge arresters, current-voltage non-linear resistors are being sought that have not merely high resistance but also reduced diameter.
However, when a current-voltage non-linear resistor is increased in resistance or reduced in diameter, the energy generated per unit volume when performing surge energy absorption becomes larger. Further improvement in performance is therefore required by example improving the energy absorption capability (or temporary overvoltage energy absorption capability) or by reducing to a minimum the generated energy, by improvement in the non-linear resistance characteristic. Also, a current-voltage non-linear resistor has the characteristic that its resistance decreases with rise in temperature. Consequently, if the current-voltage non-linear resistor is made smaller in diameter, as the temperature rises due to generation of heat produced by Joule heating after absorption of surge energy, the leakage current is increased, with the risk that thermal runaway of the surge arrester may be caused by the mains-frequency current after absorption of surge energy. For these reasons, the size of conventional current-voltage non-linear resistors was restricted and their range of application was limited. In the case of conventional surge arresters, it was therefore difficult to achieve sufficient reduction in diameter.
Also, although, in the case of conventional current-voltage non-linear resistors, the technique may be adopted of employing solely rare earth raw materials having a grain growth suppressing effect to prepare very fine-grained powder beforehand, if very fine-grained powder prepared beforehand is employed solely for a portion of the raw material, when this is mixed with another raw material, aggregation of the grains of the very fine-grained powder takes place. Uniform mixing of the powder was thus difficult to achieve. This impaired the beneficial effect of the use of fine grains, with the result that the properties of the current-voltage non-linear resistor that was finally manufactured became non-uniform, leading to the problem that the target performance could not be obtained.